Several industrial scale processes such as biomass degradation are known to produce dilute aqueous solutions comprising low amounts of furfural and organic acids such as carboxylic acids. Due to environmental reasons recovery of furfural and organic acids has become increasingly interesting. These economically valuable by-products have typically been recovered by distillation or extractive distillation, which are effective but energy consuming and technically challenging processes due to formation of azeotropes or stable emulsions rendering processing uneconomical or providing the product in an undesirable form, such as too dilute solution, which is difficult to use in further processes.
Separation of various chemicals may be based on liquid-liquid extraction processes. Even carboxylic acids have been separated from dilute aqueous solutions with extraction solvents insoluble or slightly soluble in water, or with solvent mixtures. However, the efficiency of extraction agents is typically not satisfactory enough to yield pure components or the extractant binds an acid so strongly that recovery of the acid becomes difficult.
Carboxylic acids and furfural are formed as products in various biomass hydrolysis processes. Economical recovery of these components from typically dilute water mixture obtainable from these processes involves liquid-liquid extraction as a first stage followed by multiple distillation units to recycle extraction solvent and separate extracted components as pure products. Both physical and reactive solvents as well as their combinations have been used to extract carboxylic acids and furfural. The published single extraction solvent systems for these compounds either extract carboxylic acids with good yield or extract furfural but have not been able to extract both types of chemicals simultaneously in good yield without side effects. Physical extraction solvents like hydrocarbons, ketones and esters have usually unfavourable extraction yield for formic acid. Reactive extraction solvents, such as trialkyl amines, are typically specific for carboxylic acids but extraction of furfural and separation of acids from strong amine-acid complexes as an additional phase are problematic. Trialkyl phosphine oxides, e.g. Cyanex 923, are good extraction chemicals for carboxylic acids and furfural but the formed complex with acids, especially with formic acid, is strong and needs separation by distillation.
U.S. Pat. No. 2,437,519 discloses extraction of lower aliphatic acids such as formic acid and acetic acid from dilute aqueous solutions using tetrahydrofurane (THF) or its derivatives, such as 2-methyltetrahydrofurane (2-MTHF), as the extracting solvent. U.S. Pat. No. 2,437,519 further teaches that it is preferred to add to the solvent an amount of a third substance, practically insoluble in water, for instance a hydrocarbon like benzene, to reduce the amount of water dissolved in the aqueous layer of the extracting solvent and acid. The concentration of extracted acid is thus increased and the solubility of the extracting solvent in the aqueous layer is reduced. The extract that is obtained is treated according to any known methods for recovering therefrom its acid content in anhydrous conditions. It may be subjected, for instance, to a distillation during which azeotropic dehydration of the extract takes place with the extracting solvent playing the part as a water entrainer and being separated from the anhydrous acid. The document, however, does not discuss mixtures including other organic compounds, such as furfural.
The applicant's former patent application WO 2009/130387 relates to a process for the recovery of formate salt from biomass. An aqueous liquid mixture containing levulinic acid, formic acid and possibly furfural is subjected to a liquid-liquid extraction step, followed by recovery of furfural, formate salt and levulinic acid or levulinic salt. In the disclosed process a mixture containing formic acid and levulinic acid and optionally furfural is (i) subjected to liquid-liquid extraction by employing an extracting agent whereby an organic phase comprising the extracting agent, formic acid, levulinic acid and optionally furfural, and an aqueous phase comprising essentially water, preferably further containing inorganic acid(s), are obtained; (ii) optionally, furfural is separated and recovered, preferably by distillation and gravitational separation, from the organic phase; (iii) formic acid is recovered by distillation as concentrated acid from the organic phase. The organic phase contains formic acid and levulinic acid from step (i) or optionally from step (ii); and (iv) levulinic acid is recovered from the organic phase. Preferred extracting agents are tertiary amines, secondary or tertiary amides, tertiary phosphine oxides, tertiary phosphates, C5-C12 fatty acids, C8-C12 fatty alcohols and alkyl urea derivatives.
The reactive extraction solvents e.g. trialkyl amines of WO 2009/130387 are typically specific for carboxylic acids but their ability to extract furfural depends on feed pH and they form very strong amine-formic acid complexes. Often these complexes separate as second organic phase. Trialkyl phosphine oxides, such as Cyanex 923, are good extraction chemicals for carboxylic acids and furfural but the formed complex with acids, especially with formic acid, is strong and has to be separated by distillation.
The separation of valuable compounds such as organic acids and furfural from biomass processes has been achieved by evaporation of furfural-water mixture and treatment of the acid containing aqueous waste either with an extraction or with a distillation entreiner. These treatments work usually much better with acetic acid than with formic acid.
An article by Xing R., et al. “Production of furfural and carboxylic acids from waste aqueous hemicellulose solutions from the pulp and paper and cellulosic ethanol industries”, Energy Environ. Sci., 2011, 4, 2193 discloses a process to produce furfural and co-products of formic and acetic acids from waste aqueous hemicellulose solutions using a continuous two zone biphasic reactor. Furfural is produced in a two-step process consisting of hydrolysis of xylose oligomers followed by the dehydration of xylose monomers and then extraction of the furfural into an organic solvent. The aqueous hemicellulose solution (aqueous phase) is saturated with sodiumchloride (NaCl). In the process NaCl-pretreated tetrahydrofuran (THF) is primarily used as the organic phase due to its great affinity for furfural, low boiling point, and ease of separation from water.
THF is pretreated with NaCl, because pure THF is totally soluble in water. For the same reason the aqueous phase is saturated with NaCl. Addition of NaCl does not improve THF as an extractant but it helps in the separation of the phases by salting out. The use of NaCl in an extraction of industrial scale is however not economically viable. The reuse and disposal of the formed salt solution with traces of water-miscible organic components becomes complicated.
As disclosed above there is a need for a more economical and efficient process to separate and recover carboxylic acid(s) and furfural simultaneously from a dilute aqueous mixture with a single extraction solvent.